The present invention relates to tribocharging of webs, and more particularly, to a web dispenser that imparts an electrostatic charge to a web as the web is dispensed.
Electrically charged sheets of plastics such as polypropylene are well known in the art. Such sheets are normally created by passing a thin sheet of plastic between two electrodes that impart a charge to the sheet. Such charged sheets are used as a print medium for advertising, and the like, because these sheets will cling to a vertical surface without the aid of glue. In advertising or poster applications, sheets having a deeply imbedded charge that remains for an extended period of time (weeks or months) are preferred. Such sheets are the subjects of a number of U.S. patents. For example, U.S. Pat. Nos. 5,989,685 and 5,904,985 describe printing articles consisting of a sheet of electrically charged plastic connected to a backing sheet that provides stability during the printing process and also increases the lifetime of the charge.
Electrostatically charged plastic sheets are also useful as a xe2x80x9cwhite boardxe2x80x9d for writing with markers. Sheets of electrostatically charged plastic for use in such applications have been known for some time. For example, U.S. Pat. No. 5,207,581 describes an easel with a roll of electrostatically charged plastic in a dispenser attached to the easel. In this arrangement, a section of charged plastic is withdrawn from the dispenser and sticks on the easel. After the user finishes writing on the easel, the sheet may be removed from the easel and transferred to a wall or other surface where it will stick because the sheet remains charged even after it is removed from one surface and moved to another surface.
Such sheets have also been used as a means for covering a surface to improve the cleanliness of the surface. For example, electrically charged sheets of plastic have been used as a covering on the walls of a clean room. In addition to sealing the walls, the sheets collect dust particles. Accordingly, these sheets improve the quality of the clean room environment. To clean the walls, the sheets are merely removed and replaced with fresh sheets, which requires substantially less work than that required to wash the walls.
These sheets have also been suggested as a protective covering for surfaces. For example, these sheets can be used to cover a portion of the walls in a child""s room on which the child wishes to write or draw. When the child is finished, the sheets can be easily removed. Since the sheets are typically made of a plastic such as polypropylene, the underlying wall is protected from the marker fluid.
For such applications to be commercially viable, the cost of the sheets must be quite low. While inexpensive thin film polypropylene sheets are commercially available, the cost of charging the sheets represents a significant fraction of the cost of the final product. When sold in precharged form, the charge must last for a period of time that is much longer than the time the sheet will be on the surface, since the sheets will be in transit or inventory for months, not to mention the time spent in the user""s possession prior to actually being dispensed.
If a very high charge is placed on the sheets to extend the shelf life of the product, the sheets may discharge when the user unrolls the material or tries to separate individual sheets from a stack of pre-cut sheets. Such discharges are clearly undesirable as the user may experience electric shocks. Accordingly, charging schemes in which a low intensity electric charge is deeply implanted in the plastic are preferred. Such charging, however, increases the cost of the final product.
Broadly, it is the object of the present invention to provide an improved dispenser for charging webs.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a dispenser having a reservoir for holding a web, characterized by a web dielectric constant, and a charging station for charging the web by tribocharging when the web is removed from the reservoir. The charging station has a first charging strip that includes a first material having a first dielectric constant different from the web dielectric constant and a contact mechanism for pressing web against the charging strip as the web is removed from the reservoir. The contact mechanism may include a second charging strip having a second material with a second dielectric constant different from the web dielectric constant. The web passes between the first and second charging strips in such embodiments. In one embodiment of the invention, the first dielectric constant is greater than the web dielectric constant, and the second dielectric constant is less than the web dielectric constant. In another embodiment of the invention, the first charging strip includes a third material having a dielectric constant different from the first dielectric constant, the first and third materials being arranged as an array of alternating pads such that the web is charged in stripes of alternating polarity as the web is removed from the reservoir. The reservoir preferably includes a box having a lid with a flap that overlies a portion of an outer surface of the box, the first charging strip being located between the flap and the outer surface of the box. The user presses on the flap as the web is dispensed thereby forcing the web against the charging strip. In another embodiment, the first charging strip is located on the outer surface of the box and the second charging strip is located on the flap.